I'm have 2 zimbra's servers with de same domain "example.com" behind a smart host server with postfix. Both zimbra's servers are located in remotes cities and contain accounts from the same domain "example.com". When incoming an e-mail from internet, the smart host take the desicition to relay the message to the right zimbra server who contain the destination account. Both zimbra are configured in split domain mode.

The problem is that if the smart host try to send a message to one zimbra's server and the destination account does not exist in this server, zimbra relay again the message towards the smart host (due to the split domain configuration) originating a loop instead of submit an unknown user error message.

My question is: Under zimbra split domain configuration, there is any way to permit local user relay just only for outgoing traffic from local senders (example.com domain users) and restrict that for incoming traffic relayed by the smart host in order to avoid loop?